MRCSP Phase I Geologic Characterization Report - Midwest ...

138 CHARACTERIZATION OF GEOLOGIC SEQUESTRATION OPPORTUNITIES IN THE MRCSP REGION
a figure perhaps representative of the Waste Gate Formation elsewhere
in Maryland.
Hansen (1984) noted in the Hammond No. 1 well, the formation
is “about 70 percent sandstone (sand); however each unit is relatively
thin and rarely exceeds 100 feet in thickness. In Maryland,
carbonaceous laminae and calcareous sandstones are present in the
Waste Gate, but no coaly seams or limestones have been found.”
Hansen (1982) noted rare occurrences of acritarch cysts found
within the Waste Gate Formation in the Bethards No. 1 well. A more
detailed discussion of the palynology can be found in Doyle (1982)
and Hansen (1982).
Sandstone porosities estimated from geophysical logs generally
range from 19 to 27 percent (Hansen 1982). Based on aquifer tests
of a well in Crisfield, Maryland, transmissivities of 340 to 430
gallons/day/foot and hydraulic conductivities of 4 to 5 gallons/day/
square foot were recorded (Hansen, 1984). Based on limited test
data, Hansen (1984) suggested that the Waste Gate sandstones are
likely to have relatively low permeabilities in comparison to other
Coastal Plain aquifers, perhaps on the order of 15 to 150 md.
It should be noted that younger units in the Potomac Group include
aquifers that are an important source of fresh water supply, particularly
in areas west of the Chesapeake Bay in Maryland and in some
communities on the Delmarva Peninsula. However, under much of
the Delmarva Peninsula, the deep Potomac units, including the Waste
Gate Formation, are saturated with salty water ranging from slightly
brackish to brines with salinities greater than seawater.
DISCUSSION OF DEPTH AND THICKNESS RANGES
The top of the Waste Gate Formation ranges from a depth of
about 3,500 feet at its up-dip limit to 5,670 feet near the coast (Hansen,
1984) (Figure A17-4). The Waste Gate Formation is estimated
to range in thickness from zero feet thick at its up-dip pinchout to
about 1,515 feet thick near the Delmarva coast (Hansen, 1984) (Figure
A17-5). Hansen (1984) indicate that the Waste Gate Formation
thins relatively rapidly by onlap under the Delamarva Peninsula.
The location of the up-dip edge of the unit is poorly defined because
of a lack of data, but the pinchout line is estimated to trend roughly
northeast-southwest through the middle of the Delmarva Peninsula,
based the unit’s absence in the few wells in the western part of the
Peninsula that penetrate to pre-Mesozoic basement.
DEPOSITIONAL ENVIRONMENTS/
PALEOGEOGRAPHY/TECTONISM
In early Cretaceous time, Potomac Group sedimentation began
to occur as the Piedmont and Blue Ridge provinces were uplifted.
These sediments were deposited eastward in a broad, open basin
(Glaser, 1969). This basin is referred to as the Salisbury Embayment
(formerly referred to as the Chesapeake-Delaware Embayment)
(Figure A17-3). Glaser (1969, p. 74) also indicated that deposition
probably “ began near or somewhat beyond the present coast line”
and then “…apparently migrated slowly westward toward the present
day outcrop belt” (Figure A17-2). According to Glaser (1969)
by the time Patuxent Formation sediments were being deposited,
the Early Cretaceous fall line or basin margin was located, perhaps,
only a few miles inland from the present outcrop margin.
It should also be noted that the gradient of the surface of the pre-
Mesozoic basement appears to vary, apparently increasing from
west/northwest to east on the Delmarva Peninsula. The apparent dip
of the basement surface between a well on the western side of the
Delmarva Peninsula (at Cambridge, Maryland) and the Hammond
well (central Delmarva Peninsula) is on the order of 64 feet per mile,
whereas the apparent dip between the Hammond well and Bethards
well on the eastern edge of the Delmarva Peninsula is roughly 150
feet per mile. Given the paucity of wells that penetrate into pre-
Mesozoic basement rocks in the Delmarva Peninsula, the nature of
the basement surface is not fully characterized (e.g., the extent to
which there is local relief) and it is not clear if this apparent change
in surface gradient between the two wells may be the result of pre-
Mesozoic erosion (e.g., a canyon), warping, a structural feature, or
some combination of the three. Hansen (1978) noted that the change
in gradient that occurs within about 10 to 15 miles of the coast, that
is, between the Hammond No. 1 and Bethards No. 1 wells, suggests
the presence of a tectonic hinge zone that might define the shoreward
edge of the Baltimore Canyon Trough (Figure A17-3).
Rare occurrences of marine or brackish-water fossils have been
noted within the Waste Gate Formation (Hansen, 1984) but, overall,
the majority of evidence suggests the formations originated
in a high-energy, alluvial setting dominated by fluvial channel facies
proximal to the early Cretaceous Fall Line. Hansen (1984)
noted that self-potential (SP) log signatures of the sandstones have a
blocky aspect, suggestive of braided or stacked sand-channel deposition,
rather than the well-defined, fining-upward cycles generally
ascribed to deposition by meandering streams. This suggests deposition
in a high-energy alluvial complex. In addition, Doyle (1982)
reported that samples from the Waste Gate Formation suggest deposition
in a humid tropical climatesuch as would be found in the
southern Laurasian continent during the Early Cretaceous.
SUITABILITY AS A CO 2
INJECTION TARGET OR SEAL UNIT
Porosities of the Waste Gate sandstone, as estimated from compensated
formation density logs and s electric logs, range from 19
to 27 percent (Hansen, 1982, tab. 4). In general, porosity decreases
with increasing depth (Table A17-1). In the shallowest (3,900 to
4,225 feet) of five wells penetrating the Waste Gate in Maryland,
direct pumping tests yielded sandstone permeabilities in the range
of 75 to 120 md; the Schlumberger method yielded similar results
of 63 to 122 md. In general, permeability decreases with increasing
depth, falling in the range of 16 to 122 md (Table A17-1).
Chemical analyses of the formation waters from two Waste Gate
aquifers revealed brines with chloride concentrations of 42,000
mg/l, and salinity (equivalent NaCl concentration) estimates from
electric log data ranged from roughly 25,000 to 94,000 ppm (Hansen,
1982, ttabs.. 5 and 6). Salinity increases with depth and are a
calcium/sodium chloride type. These waters are at normal hydrostatic
pressure and exhibit very lethargic to stagnant flow systems
(Hansen, 1984) (Table A17-1).
Hansen (1982, 1984) evaluated the Waste Gate Formation for potential
for extraction of chemical commodities such as commercial
brines, extraction of geothermal heat, and disposal of hazardous/liquid
waste. Perhaps most pertinent to considerations for CO 2 injection
is the hazardous/liquid waste evaluation.
Hansen (1984, p. 18) notes:
The fl uvial-deltaic Waste Gate Formation is not ideal for well
injection because of its complex sand stratigraphy. Individual aquifers
and confining beds are only locally correlative. The geometry of
each sand body is complex. With sparse well control, it is impossible
to predict unequivocally whether a potential reservoir is laterally
connected with an adjacent sand or whether it wedges out within a